Physics

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Shiv Nadar University

We present bounds on all the known universal extra dimension models from the latest Higgs search data at the Large Hadron Collider, taking into account the Kaluza-Klein (KK) loop effects on the dominant gluon-fusion production and on the diphoton/digluon decay. The lower bound on the KK scale is from 500GeV to 1TeV depending on the model. We find that the Higgs production cross section with subsequent diphoton decay can be enhanced by a factor 1.5 within this experimental bound, with little dependence on the Higgs mass in between 115GeV and 130GeV. We also show that in such a case the Higgs decay branching ratio into a diphoton final state can be suppressed by a factor 80%, which is marginally observable at a high energy/luminosity option at the International Linear Collider. The Higgs production cross section at a photon-photon collider can also be suppressed by a similar factor 90%, being well within the expected experimental reach.

World University root node

(SoNS) School of Natural Sciences

International Workshop on Future Linear Colliders, LCWS 2011

Higgs at ILC in universal extra dimensions in light of recent LHC data

We demonstrate that the recent paper by Abhinav and Panigrahi entitled 'Supersymmetry, PT-symmetry and spectral bifurcation' [K. Abhinav, P.K. Panigrahi, Ann. Phys. 325 (2010) 1198], which considers two different types of superpotentials for the PT-symmetric complexified Scarf II potential, fails to take into account the invariance under the exchange of its coupling parameters. As a result, they miss the important point that for unbroken PT-symmetry this potential indeed has two series of real energy eigenvalues, to which one can associate two different superpotentials. This fact was first pointed out by the present authors during the study of complex potentials having a complex sl(2) potential algebra. © 2010 Elsevier Inc.

Fulltext

Annals of Physics

Comment on 'Supersymmetry, PT-symmetry and spectral bifurcation'

We use conducting atomic force microscopy (AFM) in ultra high vacuum to measure the thermoelectric power of Au, Pt, and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) films. Tips coated with thick (1200 nm) Pt films or highly doped diamond film give reproducible data. The thermoelectric power of metal junctions formed with diamond tips is high but dominated by the diamond material thus making diamond tips of limited applicability in thermovoltage AFM. Pt coated tips on Au or Pt films gives small thermovoltage signal, making quantitative analysis of the thermopower on metal sample problematic. The thermovoltage AFM technique appears best suited to study organic thin films and the thermoelectric power of 1.5 nm and 2 nm thick PTCDA deposited on Au measured with Pt tips is -342 and -372 μV/K, respectively. The negative sign indicates that the lowest unoccupied molecular orbital level dominates electrical transport. © 2011 American Institute of Physics.

Journal of Applied Physics

Thermoelectric measurements using different tips in atomic force microscopy

We have investigated the photoluminescence and ultrafast dynamics of LaAlO3 crystal. The photoluminescence consists of a broad spectrum and two sharp peaks, which arise from various defect levels within the bandgap. A doublet splitting of roughly 6 nm is seen in these two sharp peaks. An Al displacement of 0.09 Å in a sublattice, which is possible because of twinning, is adequate to explain the spectral splitting. Femtosecond pump probe experiments reveal further that many of these defect levels have a few picosecond decay times while the lowest defect states have decay times longer than nanosecond to the valence band. © 2011 American Institute of Physics.

Applied Physics Letters

Defect dynamics and spectral observation of twinning in single crystalline LaAlO3 under subbandgap excitation

The electroluminescence (EL) of Er-implanted SiO2 layers containing Ge nanocrystals (NCs) was investigated and correlated with microstructural results obtained by transmission electron microscopy, Raman spectroscopy and X-ray diffraction. In case of EL, and in contrast to the behaviour of Er-doped Si-rich SiO2 known from the literature it appears that there is an inverse energy transfer from Er to Ge-related oxygen deficiency centres which are located at the surface of the Ge NCs or in the transition region between the NC and the SiO2 matrix. This is indicated by the increase of the blue-violet, Ge-related EL in presence of Er, although the Ge-related photoluminescence, which was excited by UV wavelengths non-resonant to Er, decreases at the same time. The microstructural results reveal that the maximum increase of the Ge-related EL occurs when the Ge NCs are not amorphized and/or fragmented by the Er implantation but surrounded by an Er shell. Finally a qualitative model is given explaining the different behaviour of Er-implanted SiO2 containing Ge NCs compared to the case of Si NCs by the different quality of the NC interface to the SiO2 matrix. © 2010 Elsevier B.V. All rights reserved.

Optical Materials

The inverse energy transfer between Ge nanocrystals and erbium in SiO 2 and its dependence on microstructure

We report on the electric-field-induced reversible metal-insulator transition (MIT) of the insulating LaAlO3 thin films observed in metal/LaAlO3/Nb-SrTiO3 heterostructures. The switching voltage depends strongly on the thickness of the LaAlO3 thin film which indicates that a minimum thickness is required for the MIT. A constant opposing voltage is required to deplete the charges from the defect states. Our experimental results exclude the possibility of diffusion of the metal electrodes or oxygen vacancies into the LaAlO3 layer. Instead, the phenomenon is attributed to the formation of a quasi-conduction band (QCB) in the defect states of LaAlO3 that forms a continuum state with the conduction band of the Nb-SrTiO3. Once this continuum (metallic) state is formed, the state remains stable even when the voltage bias is turned off. The thickness dependent reverse switch-on voltage and the constant forward switch-off voltage are consistent with our model. The viewpoint proposed here can provide an alternative mechanism for resistive switching in complex oxides. © 2011 American Physical Society.

Physical Review B - Condensed Matter and Materials Physics

Reversible metal-insulator transition in LaAlO3 thin films mediated by intragap defects: An alternative mechanism for resistive switching

Magnetoresistance (MR) anisotropy in LaAlO3/SrTiO3 interfaces is compared between samples prepared in high-oxygen partial-pressure (PO2) of 10-4 mbar exhibiting quasi-two-dimensional (quasi-2D) electron gas and low PO2 of 10-6 mbar exhibiting 3D conductivity. While the 3D interfaces show positive MR for all temperatures, the quasi-2D interfaces show negative MR below 30 K, suggesting the presence of magnetic centers in the quasi-2D system. Interestingly, a linear relation between MR and applied magnetic fields was observed in the 3D interfaces for an in-plane field, while in the quasi-2D interfaces for an out-of-plane field, demonstrating the dominant effect of the interface and substrate's vicinal step scattering, respectively. Furthermore, the MR with the out-of-plane field is always larger compared to the MR with in-plane field, suggesting lower dissipation of electrons from interface than defect scattering. Our study demonstrates that MR can be used to distinguish the dimensionality of the charge transport and various (defect, magnetic center, and interface boundary) scattering processes in this system. © 2011 American Physical Society.

Magnetoresistance of two-dimensional and three-dimensional electron gas in LaAlO3/SrTiO3 heterostructures: Influence of magnetic ordering, interface scattering, and dimensionality

The InAs quantum structures were formed in silicon by sequential ion implantation and subsequent thermal annealing. Two kinds of crystalline InAs nanostructures were successfully synthesized: nanodots (NDs) and nanopyramids (NPs). The peaks at 215 and 235 cm-1, corresponding to the transverse optical (TO) and longitudinal optical (LO) InAs single-phonon modes, respectively, are clearly visible in the Raman spectra. Moreover, the PL band at around 1.3 μm, due to light emission from InAs NDs with an average diameter 7±2 nm, was observed. The InAs NPs were found only in samples annealed for 20 ms at temperatures ranging from 1000 up to 1200°C. The crystallinity and pyramidal shape of InAs quantum structures were confirmed by HRTEM and XRD techniques. The average size of the NPs is 50 nm base and 50 nm height, and they are oriented parallel to the Si (001) planes. The lattice parameter of the NPs increases from 6.051 to 6.055 Å with the annealing temperature increasing from 1100 to 1200°C, due to lattice relaxation. Energy dispersive spectroscopy (EDS) shows almost stoichiometric composition of the InAs NPs. © 2011 Versita Warsaw and Springer-Verlag Wien.

Central European Journal of Physics

Optical and microstructural properties of self-assembled InAs quantum structures in silicon

We investigate complex versions of the Korteweg-deVries equations and an Ito-type nonlinear system with two coupled nonlinear fields. We systematically construct rational, trigonometric/hyperbolic and elliptic solutions for these models including those which are physically feasible in an obvious sense, that is those with real energies, but also those with complex energy spectra. The reality of the energy is usually attributed to different realizations of an antilinear symmetry, as for instance -symmetry. It is shown that the symmetry can be spontaneously broken in two alternative ways either by specific choices of the domain or by manipulating the parameters in the solutions of the model, thus leading to complex energies. Surprisingly, the reality of the energies can be regained in some cases by a further breaking of the symmetry on the level of the Hamiltonian. In many examples, some of the fixed points in the complex solution for the field undergo a Hopf bifurcation in the -symmetry breaking process. By employing several different variants of the symmetries we propose many classes of new invariant extensions of these models and study their properties. The reduction of some of these models yields complex quantum mechanical models previously studied. © 2011 IOP Publishing Ltd.

Journal of Physics A: Mathematical and Theoretical

PT-symmetry breaking in complex nonlinear wave equations and their deformations

Since discovery of graphene, layered materials have drawn considerable attention because of their possible exfoliation into single and multilayer 2D sheets. Because of strong surface effects, the properties of these materials vary drastically with the number of layers in a sheet. We have performed first-principles density functional based calculations to evaluate the electron energy loss spectrum (EELS) of bulk, monolayer, and bilayer configurations of several transition metal dichalcogenides, which include semiconducting as well as metallic compounds. Our investigation shows that the peaks in the EELS spectra move toward larger wavelengths (red shift) with the decrease in number of layers. The π plasmon peak shifts slightly by 0.5-1.0 eV, while a significant shift of around 5.5-13.0 eV is obtained for π + σ plasmon, when exfoliated from bulk to single-layer. This underscores the importance of the interlayer coupling on the loss spectra and the dielectric properties. Our results are found to be in very good agreement with the recent measurements performed by Coleman et al. (Science2011, 331, 568). © 2011 American Chemical Society.